1. Field of the Invention The present invention relates to a current control circuit, in particular a current control circuit having a current sensing circuit.
2. Description of Related Art
In recent years, cameras of mobile phones are equipped with a zooming and focusing function. Therefore, the motors that control the lens position of such cameras have been required to operate with stability regardless of the temperature change.
Firstly, the motor needs to be supplied with a stable current to ensure the stable operation of the motor. Therefore, in general, a current control circuit is used. However, in the case of current control circuits in the related art, the resistance value of the sense resistor provided in those current control circuits changes as the temperature changes. As a result, the current flowing to the motor also fluctuates. Therefore, suppressing the fluctuation of the resistance value of the sense resistor due to the temperature change has been a significant problem to be solved.
For example, current control circuits equipped with a sense MOS transistor (hereinafter, simply referred to as “sense MOS”) have been known as an IC (Integrated Circuit) capable of outputting a stable current to drive a motor for controlling a lens position.
FIG. 5 shows a current control circuit equipped with a sense MOS shown in Japanese Unexamined Patent Application Publication No. 2007-244083. Note that the circuit shown in FIG. 5 is an H-bridge type current control circuit 10. The circuit shown in FIG. 5 monitors a current flowing to the load (output current), and compares a voltage generated based on the current with a predetermined reference voltage. Then, by feeding back its comparison result, the circuit controls the output current with stability. Note that for the sake of simplifying the drawing, connection lines between the pre-driver 15 and the gates of TR2 and TR3 are omitted in FIG. 5.
As shown in FIG. 5, the current control circuit 10 includes an H-bridge circuit 11 composed of transistors TR1 to TR4, a triangular-wave generator 12, a superimposition circuit 12a, a reference DAC 13, a sense resistor SRI, a PWM comparator 14, a pre-driver circuit 15, a sense MOS 16, a transistor TR5, and an amplifier 18. Further, the current control circuit 10 drives a motor 17.
Note that the triangular-wave generator 12, the reference DAC 13, and the superimposition circuit 12a constitutes a reference voltage source. In this reference voltage source, the superimposition circuit 12a superimposes a triangular-wave signal having a predetermined frequency generated by the triangular-wave generator 12 on a reference level generated by the reference DAC 13 to output a reference signal V1.
The reference signal V1 is input to the non-inverting input terminal of the PWM comparator 14. Meanwhile, a sense signal Vs that changes according to the current flowing to the load is input to the inverting input terminal of the PWM comparator 14. The PWM comparator 14 compares the reference signal V1 with the sense signal Vs to output a PWM signal Vp.
The PWM signal Vp is input to the pre-driver circuit 15. The pre-driver circuit 15 outputs a switching signal(s) based on the PWM signal Vp. The switching signal(s) is used to perform the switching of the transistors TR1 to TR4 of the H-bridge circuit 11.
The H-bridge circuit 11 is composed of four transistors TR1 to TR4 provided between a high-potential side power supply VM and a low-potential side power supply GND. Further, the H-bridge circuit 11 can change the direction of the current flowing through the motor 17. Note that the transistors TR1 and TR3 are P-channel MOS transistors, and the transistors TR2 and TR4 are N-channel MOS transistors.
As for specific operations, the H-bridge circuit 11 lets a current flow from the high-potential side power supply VM through the transistor TR1, the motor 17, and the transistor TR4 to the low-potential side power supply GND, for example, by turning on the transistors TR1 and TR4 and turning off the transistors TR2 and TR3. On the other hand, by turning off the transistors TR1 and TR4 and turning on the transistors TR2 and TR3, the H-bridge circuit 11 lets a current flow from the high-potential side power supply VM through the transistor TR3, the motor 17, and the transistor TR3 to the low-potential side power supply GND.
Note that FIG. 5 shows an example of a state of the circuit in which: the transistor TR1 is in an On-state; the transistors TR2 and TR3 are in an Off-state; and On-Off switching control (PWM control) is performed on the transistor TR4. As a result, the current flowing from the high-potential side power supply VM through the transistor TR1, the motor 17, and the transistor. TR4 to the low-potential side power supply GND is controlled.
Further, when the direction of the current flowing through the motor is to be changed, the transistor TR3 is turned on and the transistors TR1 and TR4 are turned off. Further, On-Off switching control (PWM control) is performed on the transistor TR2 in that state. As a result, the current flowing from the high-potential side power supply VM through the transistor TR3, the motor 17, and the transistor TR2 to the low-potential side power supply GND can be controlled. Note that in this case, some necessary switching is performed so that the gate voltage of the transistor TR3 is applied to the gate of the sense MOS 16 and that the voltage on the right side of the motor 17 (in the drawing) is input to the non-inverting input terminal of the amplifier 18.
Note that an assumption is made that each of the transistors that are turned on, among the transistors TR1 to TR4, has a voltage equal to the potential difference between the high-potential side power supply VM and the low-potential side power supply GND between its gate and source. That is, assume that those transistors are in the completely-on state (Fully-On state).
Further, the circuit shown in FIG. 5 is explained as an example where one H-bridge circuit 11 is provided to drive one load (motor 17). However, in the case of driving a stepping motor or the like, the circuit needs to be equipped with two or more than two H-bridge circuits.
Next, a current flows to the sense MOS 16 based on the current value supplied to the motor 17. A sense signal Vs is generated based on the current flowing to the sense MOS 16 and the resistance value of sense resistor SR1. Note that the amplifier 18 controls the transistor TR5 such that the drain voltage of the transistor TR1 becomes equal to the drain voltage of the sense MOS 16. In this manner, the related art controls the current flowing to the load.
Note that, as for the reference voltage (reference signal V1), various voltages including ones shown below may be used. In a case where the reference voltage is formed by a DC voltage alone (FIG. 2A), the output current flowing to the motor 17 has a waveform shown in FIG. 2B. Further, waveforms of reference voltages that are formed by superimposing a triangular wave for driving a PWM on a DC voltage are shown in FIGS. 3A and 3B. Further, waveforms of reference voltages that are formed by superimposing a triangular wave for driving a PWM on a half-wave SIN wave voltage based on the output signal of the reference DAC 13 are shown in FIGS. 4A and 4B. As shown in FIGS. 2A and 2B to FIGS. 4A and 4B, the output current waveforms to the load corresponding to the respective reference voltages have a triangle-shaped ripple component. In the case of FIG. 2B, the ripple component of the output current has a frequency according to the load (e.g., motor 17) and the resistive component. Further, in the cases of FIGS. 3B and 4B, the ripple component of the output current has a frequency according to the triangular wave.
Further, Japanese Unexamined Patent Application Publication No. 2003-204247 discloses a variable resistance circuit using a MOS transistor as a variable resistor. The circuit shown in Japanese Unexamined Patent Application Publication No. 2003-204247 can adjust a current ratio between two constant current sources (constant current sources 12 and 13 in Japanese Unexamined Patent Application Publication No. 2003-204247). That is, the circuit shown in Japanese Unexamined Patent Application Publication No. 2003-204247 controls the resistance value of a transistor 6 (transistor 6 in Japanese Unexamined Patent Application Publication No. 2003-204247) and the resistance value of a transistor 8 (transistor 8 in Japanese Unexamined Patent Application Publication No. 2003-204247), which is an equivalent resistor of the transistor 6, by adjusting the current ratio between two constant current sources.